Best coral eater

What is the best coral eater you know of? I will be plumbing a small tank (20-30g ) to my main display to house a fish/invert to remove a hairy mushroom infestation I have. My plan is to move over a couple rocks at a time while also doing manual removal in the main tank. I don’t want new rock and don’t wish to kill off the other life I have on my existing rock. I was thinking some kind of butterfly, but I know they can be hit or miss.
 
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